As a manager and entrepreneur we had some people that were intellectually bright, masters of cognitive ability, but were miserable and made everybody miserable.
As bright as they were, they had their partners or children abusing them, or cheating. It was clear that they were not bright in other areas of their life.
The problem was when they projected their anger, frustration or cynicism on other members of the team.
This is a red line for me, I learned to never tolerate this and it is one of the best thing you could do for your team.
Emotions are addictive and contagious. It you let negative emotions of one member go against the team, soon the entire team will react and escalate(people work this way, hey he insulted me so I double the insult back and so on).
In lots of jobs, emotional intelligence is not that important, specially when you can "outsource" it from the workers to managers but on sales it is an essential ability.